Climate Changes Linked to Fall of Roman Empire

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A prolonged period of wet weather spurred the spread of the
Bubonic plague in medieval times, according to a new study. And a
300-year spell of unpredictable weather coincided with the
decline of the Roman Empire.

Climate change wasn't necessarily the cause of these and other
major historical events, researchers say. But the study, which
pieced together a year-by-year history of temperature and
precipitation in Western Europe, dating back 2,500 years, offers
the most detailed picture yet of how climate and society have
been intertwined for millennia.

With a look to the past, the work may help society better prepare
for climate change in the future by informing public policy
decisions about water management and other resources.

"We need to have a better understanding about the ancient climate
system and its variability to understand the modern situation,"
said Ulf Büntgen, a paleoclimatologist at the Swiss Federal
Research Institute in Zurich. "It does not provide any
predictions. But it helps us take it as something to be
considered."

Büntgen and colleagues collaborated with archaeologists to amass
a database of more than 9,000 pieces of wood dating back 2,500
years. Samples came from both live trees and remains of buildings
and other wooden artifacts, all from France and Germany. By
measuring the width of annual growth rings in the wood, the
researchers were able to determine temperature and precipitation
levels on a year-by-year basis.

To get annual temperatures, they measured rings in high-altitude
conifer trees, which grow faster in warmer summers and slower in
colder years. To gauge precipitation, they looked at tree ring
widths in lower-elevation oaks, which grow faster in years with
higher levels of rainfall. Other techniques allowed them to
figure out exactly which year each ring represented.

Overall, analyses showed that the degree of climate change
occurring now is unprecedented in the last 2,500 years, Büntgen
and colleagues report today in the journal Science.
Because the data correlated weather patterns with exact years,
the researchers were also able to zero in on specific moments in
history.

Again and again, the data suggest, climate has impacted culture
in dramatic ways. Unusually extreme and frequent shifts in
weather patterns between 250 and 550, for example, coincided with
a period of exceptional upheaval in Europe's political and
economic situations.

As weather patterns stabilized again between about 700 and 1000,
on the other hand, societies began to thrive and grow in the
countryside of northwest Europe. Around the same time, Norse
colonies sprang up in Iceland and Greenland.

Climate also seems to have played a role in the epidemic of Black
Death, which killed about half the population of Central Europe
by 1347. For decades leading up to the outbreak, the new study
found, wetter summers and a major cold snap corresponded with the
onset of a Little Ice Age. Those conditions may have contributed
to widespread famine and overall poor health, predisposing people
to catch the plague.

Another particularly cold dip in the early 17th century
corresponded with the Thirty Years' War, a time when many people
abandoned Europe and migrated to America.

"It's not that there was a war because it was cold," Büntgen
said. "But the conditions were not helpful. Society was already
affected a lot by this political turmoil, and they got additional
suffering from cold summer temperatures."

Together, the findings offer new and extraordinary precise lines
of evidence for understanding the history of human societies,
said David Stahle, a geoscientist at the University of Arkansas,
Fayetteville.

"When they say 536 A.D., which is an exceptional year because of
what was likely a massive volcanic eruption, they don't mean 535
A.D. or maybe 534 A.D.," he said. "They mean 536 A.D. without a
doubt. It's nailed."

Correlations don't prove anything, he added, and plenty of
droughts, cold snaps and other climate events are associated with
no major cultural events at all. But the findings help show how
climate has acted as one of the many factors that have altered
people's lives. Now, we can see that happening in periods long
before meteorological instruments tracked every high and low.

"We live in a sea of coincidence and oughtn't jump every time the
data twitch, but this is an enormous quantity of data spanning
the heart of Western Europe," Stahle said. "It's not a reach to
say these extreme and prolonged climate activities could have
affected the trajectory of social evolution."